The present invention relates to a new and improved construction of radiation detector which is of the type embodying at least one radiation sensitive element which delivers an electrical signal in accordance with the electromagnetic radiation impinging thereat, and this invention also relates to a new and improved method of operating such radiation detector.
Such radiation detectors serve for the conversion of electromagnetic radiation in the optical, infrared or ultraviolet range into electrical signals. This conversion can be accomplished in different ways, for instance by means of the internal photoelectric effect of semiconductor materials of photoresistors, photodiodes, photoelements or phototransistors, or by the secondary effect of the heat which is formed due to the absorbed radiation in the element, wherein there can be then used for the transducing or conversion of the heat into an electrical signal, for instance the resistance change of a thermistor or the charge displacements in a polarized, pyroelectric crystal or plastic. Such radiation detectors are known in numerous constructions and are extensively used.
A drawback of such state-of-the-art radiation detectors is that a noise signal is superimposed upon the electrical output signal, this noise signal placing a lower limit upon the radiation detection. This noise signal is particularly then detremental when the radiation signal which is to be detected has a similar course, for instance if individual radiation pulses are to be detected at a certain spacing from one another which cannot be distinguished from the noise signal. When using such radiation detectors in passive infrared radiation-burglary detectors or in optical flame detectors, it is therefore possible for there to be simulated by the unavoidable noise signal an alarm signal, although in fact there is no real cause for an alarm. The sensitivity of such equipment therefore cannot be improved without limits, rather must be chosen such that there is only detected radiation which is appreciably more intensive than the noise signal.
In order to avoid this drawback it has already been proposed to employ two separate, but similar radiation detectors with separate amplifiers and threshold value switches, which are connected such that an ouput signal only then appears when both channels simultaneously carry a signal. The operation of this system is predicated upon the assumption that the probability that both channels will simultaneously deliver an identical noise signal is extremely small. What is disadvantageous with this equipment is that both radiation detectors do not always receive the same radiation, i.e., their output signals can have a slightly different course. Therefore, it can happen that with such combinbination of two separate radiation detectors, while it is possible to extensively suppress false alarms, nonetheless on the other hand in certain cases it can happen that no output signal is transmitted notwithstanding the presence of a cause for alarm. Apart from this disadvantage it is further to be appreciated that with such combination of two radiation detectors the entire circuit expenditure is more than doubled, something which is totally unacceptable for certain fields of application.